1. Field of the Invention
This invention relates to the use of high density salt type aqueous fluids in wellbores, and it relates more particularly, to the introduction into the wellbore of these fluids in a solids-free condition.
2. Description of the Prior Art
Over the years, special aqueous brine solutions have been used in wellbores employed in the production of petroleum. These solutions have been used as both drilling, completion and packer fluids especially in deep wells subject to high formation gas pressures at elevated temperatures. The use of high density brines as well fluids in drilling, completion and packer functions is a well accepted practice in the oil field. These brines can be formed of the sodium, calcium and zinc salts with chloride and bromide. These aqueous fluids may include corrosion inhibitors and other salts such as soda ash. The density of these salt type well fluids depends in the particular salt, or mixture of salts, and their concentration in the aqueous well fluid. Usually, these salt type well fluids have a density in the range of between about 8 and 17 pounds per gallon.
The salt type well fluid should be solids-free in its use as a well fluid. If there are solids in a drilling or completion fluid, they can cause serious injury to a producing formation by plugging of the pore spaces therein or even of the perforations and channels provided to induce fluid flows between the formation and well bore. If there are solids in a packer fluid, the solids will precipitate with time upon the packer. As a result, these solids deposits make it difficult to disconnect the tubing from the packer with a resultant costly well workover.
The high density salt type well fluids could be prepared at the wellsite by dissolving the prescribed amount of salt into the aqueous phase, which phase is principally water but can include various inhibitors for preventing pitting, corrosion, etc. The mixture is circulated or agitated until there were no undissolved solids. Naturally, the problems of adding salts to be dissolved in the aqueous well fluid became progressively more severe as the density increases, both in time, manpower and equipment requirements.
At present, several vendors will deliver to the wellsite the prepared high density salt type aqueous well fluids of a desired density and combination of selected ingredients. It is desired that these well fluids are clean and free of solids. However, the delivery of these well fluids usually requires several changes in containers. For example the well fluid is moved from the vendor tanks to truck transport, offshore supply boat and rig storage tanks. In most circumstances, the well fluid becomes contaminated by undesired solids, including residual water wetted solids and/or oil based drilling mud, weighting agents such as barite, rust, salt, silt and sand, and other undissolved materials. Contaminating liquids such as mud bases, lubricants and diesel fuel can also be present in the well fluid. Usually, these contaminating liquids are occluded or absorbed on the undissolved solids.
If the amount of solids in the well fluid were small in amount, the rig equipment may be used for their removal usually in a stepwise flow pattern through cartridge filters. However, the costs of manpower and rig time in filtering the well fluid is usually prohibitive (e.g., $100,000 per each work shift) unless the solids are (1) less than 0.01% by weight of the well fluid, (2) granular, and (3) not gelatinous as is usually the case with bentonite mud contamination.
Contamination of well fluid by drilling mud components is most common since the well fluid is usually handled at the rigs in parts of the mud system. The mud system usually suffers contamination during washing of the cased well bore to remove residual mud and cement solids immediately prior to the introduction of the high density salt-type completion/packer well fluids. Only a small amount of the wash liquid needs to be combined with the well fluid so that its solids content becomes excessive. Then, the well fluid must be treated to remove these solids. Any residual solids must be less than 5 microns in maximum dimension otherwise they cause formation plugging.
As mentioned, the use of cartridge filters is impractical on other than very low solid contents in the well fluid. Further, rig time in equipment and manpower is restricted and available only for critical operations, namely optimum drilling of the well bore. As a result, well fluid with large solids contamination must be either discarded or returned to some facility for purification. Since the well fluid is very expensive (e.g., $300-$900 per barrel) it cannot be discarded. Furthermore, the well fluid must be carefully handled so as not to be spilled or wasted because environment injury occurs from strong aqueous brines.
The present invention is a process for removing solids substantially completely from salt type aqueous well fluids using a minimum of chemicals and simple procedural steps; and, the solids removal is accomplished in only one or two hours using rig equipment and manpower.